This invention relates to code-division multiple-access (hereinafter, CDMA) spread-spectrum communications, and more particularly to a method and apparatus for increasing the number of users who can transmit simultaneously without increasing tile bandwidth requirement.
CDMA is a digital communication system that allows multiple users to communicate in the same frequency band. Briefly, each user's data is modulated by a different spreading code having a rate of N chips per data symbol (N being an integer greater than one), and all user's data are transmitted on the same carrier frequency. A receiver can recover a particular user's transmitted data by demodulating the received signal with that user's spreading code.
If the spreading codes are all mutually orthogonal over each symbol duration, then the demodulated signals will be free of interference. The number of mutually orthogonal spreading codes available depends on the chip rate N: the higher the value of N, the more orthogonal codes there are. If the spreading codes are only approximately orthogonal, then the number of different codes that can be used before interference causes an unacceptably high error rate depends similarly on N. In either case, higher values of N allow more users to transmit simultaneously; that is, higher values of N provide more user channels.
Accordingly, a simple way to accommodate more users in a CDMA system is to increase the chip rate. Unfortunately, this also increases the bandwidth of the transmitted CDMA signal. Operators of CDMA systems that have a fixed bandwidth allocation, such as digital cellular telephone systems, face tile dilemma of needing to increase their user capacity without being able to increase their bandwidth.